Make histograms pickleable

[Mirochill]

Summary

• Add explicit pickle state handling for HdrHistogram so ctypes-backed encoder/count storage is rebuilt on unpickle.
• Preserve constructor settings, non-zero counts, min/max/total values, timestamps, tag, and conversion ratio across round trips.
• Add a pickle round-trip regression covering recorded counts and metadata.

Fixes #51

Validation

• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD
• git show --check --stat --oneline HEAD
• Not run locally